The present invention relates to a shift control system for an automatic transmission, and more particularly to a shift control system arranged to prevent an automatic transmission from generating an interlock during a shift executed by an engagement of one friction element and a disengagement of another friction element under a low temperature condition.
An automatic transmission shown in "A Service Manual of RE4R0lA type Automatic Transmission (A261C07)" published by Nissan Motor Co., Ltd. is arranged to execute a shift by engaging a friction element and by disengaging another friction element. The engagement of the friction element to be engaged is quickly executed even if a viscosity of hydraulic oil of the automatic transmission becomes high due to a low temperature because the pressurized hydraulic oil is supplied to the friction element to be engaged. In contrast, if the viscosity of the hydraulic oil is high under the low temperature condition, the disengagement of the friction element to be disengaged is delayed according to the increase of the viscosity of the hydraulic oil since the disengagement operation is executed by simply draining the hydraulic oil from the friction element to be disengaged. This time delay of the disengagement will degrade a shift feeling applied to a driver. A Japanese Patent Provisional Publication No. 8-233077 has proposed a shift control system arranged to forbid or disable a manual shift when a hydraulic oil temperature is low, in order to avoid a driver from feeling the degraded shift feeling of the manual shift under the low temperature condition.